Whilst the idea of a 'private' blockchain is *almost* ridiculous, and a non-sequitur, it isn't quite; google introduced Certificate Transparency about a decade ago, and from my limited understanding, it is a pretty useful construct, as it allows key revocations/updates to be propagated much more efficiently.

You don't have to call it a 'blockchain' but it's a very similar design/protocol.

As CT descriptions make clear (see certificate.transparency.dev ), there should be some element of distributedness, though, for that construct to have value. I don't know if or how that applies to protonmail. Without that, you can still have non-repudiable update and public transparency, but it's harder to see much value. A bit tricky.